An electric cord typically employs a structure using copper wire for the core, and covering the outer periphery thereof with an insulator, and is unable to expand and contract. Although typical examples of an expandable electric cord include curl cords used in fixed telephones and the like, these are typically thick and heavy.
On the other hand, as an example of technology relating to an expandable electric cord, a method for using an elastic long fiber as a core and coiling a metal wire around the periphery thereof is disclosed in Japanese Examined Patent Publication No. S64-3967, which states that it is necessary for the relationship between the converted diameter (Ld) of the elastic long fiber and the converted diameter (Lm) of the metal wire to satisfy the expression Ld/Lm≧3 (the definition of converted diameter and calculation method are described later), and that in the case of deviating from this range, expansion and contraction are either not demonstrated or it is not possible to form a stable loop, thereby preventing the obtaining of a satisfactory expandable cord.
In addition, Japanese Patent No. 3585465 discloses technology for braiding a metal wire around an elastic long fiber and covering by braiding an insulating fiber around the outer periphery thereof. It is also described as an application thereof that this technology can be used to transmit electrical signals such as those of a headphone using this expandable cord. Namely, this technology transmits weak current. Upon closer examination of the contents, an example is given in which a metal wire having a diameter of about 0.06 mm is braided onto an elastic long fiber having a diameter of about 0.8 mm. Although it is not disclosed as to how many metal wires are used for braiding, with reference to the drawings contained in this patent publication, when calculated in the case of using 16 metal wires, the converted diameter of the metal wire becomes 0.24 mm, and the relationship between the converted diameter of the elastic long fiber and the converted diameter of the metal wire (Ld/Lm) becomes Ld/Lm=0.8/0.24=3.3, thus exceeding 3.
Moreover, Japanese Unexamined Patent Publication No. 2004-134313 discloses technology in which a conductive wire is coiled in a helical form around an expandable core, and then a plurality thereof is gathered and covered in a cord-shape. According to a disclosed example of this patent publication, it is described that a conductive wire composed of a plurality of enamel wires having a diameter of 0.03 mm are coiled in a helical form around an 840 denier polyurethane elastic long fiber. The converted diameter of the 840 denier polyurethane long fiber based on the specific gravity of polyurethane of 1.2 becomes Ld=0.03 mm. Assuming that 9 enamel wires having a diameter of 0.03 mm were used, then the converted diameter of the enamel wires becomes 0.09 mm, and the relationship between the converted diameter Ld of the elastic long fiber and the converted diameter Lm of the metal wire in this patent publication as well becomes Ld/Lm=0.32/0.09=3.6, again exceeding a value of 3. In addition, it is described that an object of the invention of this patent publication is to provide an expandable electric cord capable of being applied to various types of signal cords, indicating it to be an expandable electric cord that handles weak current.
All of the technologies disclosed in these patent publications substantially consist of coiling a conductor wire directly around an elastic long fiber, and as long as they do not satisfy the expression Ld/Lm≧3, are unable to realize expansion and contraction with respect to the rigidity of the conductor wire, or are unable to be coiled stably or form a uniformly looped shape as a result of being unable to completely oppose the elasticity generated during coiling of the elastic long fiber. Although technologies comprising the covering of an elastic long fiber with an insulating fiber are also disclosed, this sheath is provided for the purpose of reinforcement to prevent severing of the metal wire, and is not provided for the purpose of increasing the coiled diameter.
On the other hand, the prerequisites required of electric power cords include low electrical resistance and low generation of heat even when carrying a large current. The electrical resistance value is in a relationship of being inversely proportional to cross-sectional surface area for a given material, and conductor wires having a large cross-sectional area are required to produce expandable cords for electric power applications.
An expandable electric cord capable of carrying a desired current can be produced by fabricating in accordance with the technology disclosed in the aforementioned Japanese Examined Patent Publication No. 64-3967. However, since it is necessary to use a conductor wire having a large converted diameter in order to carry a large current, even in the case of using a copper wire considered to be the most common form of conductor wire, it is necessary to satisfy the expression Ld/Lm≧3, thus requiring the use of an elastic long fiber having a large converted diameter.
Since an elastic long fiber having a large converted diameter has a large cross-sectional area and expresses strong elasticity, the expandable electric cord able to be obtained from such an elastic long fiber was such that it could only be stretched by pulling with considerable force.
On the other hand, robots have advanced considerably in recent years, which are capable of demonstrating various forms of movement. The wiring employed in such robots is required to have a large allowance for movement, and there are many cases in which this presents problems in terms of equipment design and practical use.
In addition, the power current in the latest humanoid robots is wired to operate terminal motors through multiple degree-of-freedom joints, thus creating a need for increasing the degree of freedom of wiring in these multiple degree-of-freedom joints.
Moreover, in the field of industrial robots as well, development is actively proceeding on robotic hands and the like, thus creating a demand for expandable electric cords capable of carrying not only low current but also large current for operating terminal motors, while also having heat resistance enabling them to be used even in high-temperature environments at factories.
Expandable electric cords and wires are also disclosed in, for example, Japanese Unexamined Patent Publication No. 2002-313145 and Japanese Unexamined Patent Publication No. 61-290603 in addition to the patent publications previously listed. Moreover, as an example of an electrically conductive elastic composite yarn, a technology for compounding elastic fibers and metal wire is disclosed in Japanese Unexamined Patent Publication No. 2006-524758. Each of these technologies uses organic elastic fibers exemplified by polyurethane elastic fibers, and is only suitable for applications involving the carrying of weak current in room temperature environments.
On the other hand, although there are various technologies relating to industrial robot cables including Japanese Examined Utility Model Publication No. 63-30096 relating to curling for the purpose of enhancing bendability, Japanese Examined Patent Publication No. 3-25494 relating to the composition, bendability and strength of copper wire, Japanese Unexamined Patent Publication No. 5-47237 relating to a polyether- or polycarbonate-based polyurethane elastomer sheath, and Japanese Patent No. 3296750 relating to a multiconductor twisted wire composed of polyamide and polyurethane, these cables do not have expandability and were unsatisfactory for use as wiring for the joints of robots demonstrating a diverse range of movement.    Patent Document 1: Japanese Examined Patent Publication No. 64-3967    Patent Document 2: Japanese Patent No. 3585465    Patent Document 3: Japanese Unexamined Patent Publication No. 2004-134313    Patent Document 4: Japanese Unexamined Patent Publication No. 2002-313145    Patent Document 5: Japanese Unexamined Patent Publication No. 61-290603    Patent Document 6: Japanese Unexamined Patent Publication No. 2006-524758    Patent Document 7: Japanese Examined Utility Model Publication No. 63-30096    Patent Document 8: Japanese Examined Patent Publication No. 3-25494    Patent Document 9: Japanese Unexamined Patent Publication No. 5-47237    Patent Document 10: Japanese Patent No. 3296750